For a number of years, active Doppler radars have been used to detect incoming missiles, which are fired at a platform, and to cue appropriate countermeasures. One of the major problems to eliminate false alarms on ground clutter is to be able to ignore the returns from objects on the ground that are reflecting back the radar pulses to the platform. The reflected signals from the ground clutter exhibit a Doppler shift reflecting the velocity or speed of the aircraft that is overflying the region. In the past, this was accomplished by elimination or removal of signals having a Doppler shift equal to or lower than the maximum speed of the aircraft. This meant utilizing analog crystal filters having a fixed band pass.
If, for instance, the platform was flying at a maximum of 600 knots, then it was the purpose of these filters to filter out anything that had a Doppler frequency shift indicative of movement equal to or less than the 600-knot ground speed of the aircraft. Normally, since incoming missiles would approach the platform at speeds much higher than the maximum speed of the aircraft relative to the ground, filtering out Doppler shifts indicating speeds equal to or less than the maximum speed of the aircraft was effective to eliminate the ground clutter from detection or contention in determination of the presence of a threat.
The problem with such fixed analog filters is that tailoring the response to reject ground clutter utilizing the maximum speed of the aircraft denied the flexibility required when the aircraft reduced its speed from its maximum value to some lower value.
For instance, if the filters were designed to reject Doppler shifts equal to or lower than 400 knots, when the aircraft was flying at 100 knots, any Doppler returns indicating speeds between 100 knots and 400 knots would be rejected. Thus an incoming missile with a closing velocity between 100 and 400 knots would be ignored.
In short, there was a necessity for providing an inexpensive system to be able to adaptively change what is referred to herein as the “clutter line,” which refers to the Doppler shift associated with the actual ground speed of the aircraft rather than its maximum speed. This type of flexibility would result in the detection of incoming missiles or threats having a Doppler shift associated with a speed or velocity less than the maximum speed or velocity of the aircraft.
More particularly, Doppler radar detection systems are located on moving platforms, which are utilized to transmit pulses into a scanned area and to receive back reflected energy. The energy includes energy that may be reflected from a high-speed missile but more often includes back clutter, which is energy that bounces off the ground and is reflected back with a Doppler shift reflecting the relative motion of the aircraft to the ground.
The problem is to be able to detect incoming missiles approaching the platform and to be able to deploy countermeasures. It is noted that the incoming missile has a Doppler shift associated with its approaching velocity. If that velocity is above the velocity associated with the clutter, then one can easily filter out the clutter. To do this, analog filter crystals have been employed. However, these filters are very expensive, heavy and have fixed filter characteristics.
Note the analog crystal filters define a clutter region to be rejected such that the remaining energy, if it is above the Doppler frequency of the clutter, would then be detectable.
Active radar systems in the past also utilized a number of different range gates so as to be able to limit for consideration only specific ranges from the platform. Each of these range gates would then be assigned different clutter filters, with the range gate basically defining where the platform thinks that the incoming missile is.
Thus in the past there were two types of filtration to eliminate false alarms. One was range gating such that the system would eliminate consideration of returns from objects at ranges other than a predetermined range; and secondly, to process the output of the range gate to eliminate consideration of any Doppler shifts that were less than or equal to the maximum ground speed of the aircraft. In short, these filters would be high pass filters that would pass all the Doppler frequencies above the Doppler shifts associated with the clutter. Thus all energy below the clutter line would be rejected, leaving all the frequencies above the clutter line to detect fast-moving threats approaching the platform.
It is noted that ground clutter, rather than having a single Doppler shift associated with it, generally includes large structures that provide returns that indicate not a single Doppler shift but a range of Doppler shifts. This is due to the particular geometry between the platform and the ground clutter object, as well as the different angles at which the reflected radar pulse is received at the platform.
It is noted that the closing speed of a missile is in a range on the order of hundreds to thousands of meters per second, in which the highest Doppler shift is around 15 kilohertz, given, for instance, a one-gigahertz L-band radar.
It is noted that the Doppler shift could be significantly lower, namely, for instance, possibly between 3 kilohertz to 15 kilohertz depending on the particular type of missile.
In these early systems the analog crystal filters would simply eliminate Doppler shifts below some particular Doppler shift, in general that associated with the maximum speed of the aircraft involved.
However, such active radar protection could be deployed on such diverse aircraft as unmanned aerial vehicles, fixed-wing subsonic aircraft, rotary-wing aircraft or supersonic aircraft.
Choosing or designing the analog crystal filters for the maximum speed of multiple aircraft presented a problem in that they had to be redesigned for each aircraft into which they were to be installed.
Moreover, in addition to the cost of the crystal filters, they are relatively large, in some instance being 3 by 5 inches and an inch thick. With a half a dozen of such filters, they occupy a significant amount of rack space. This is a significant drawback when active radars are deployed on unmanned aerial vehicles. Moreover, the filters themselves may be several thousands of dollars each, and one is essentially stuck with a fixed velocity, assuming that the filters are cut for the maximum velocity of the aircraft. Additionally, these filters were of a sub-octave type, which limited the range/Doppler combination. A sub-octave filter is basically a filter where the ratio of the lowest frequency it passes to the highest frequency it passes is less than two. Thus they are relatively narrow-banded and were not easily adapted to aircraft of differing maximum speeds.
Additionally, inherent to the high pass band to transition band ratio of the crystal filters, is long ring or settling time, leading to an elevated false alarm rate. In order to deal with the long ring time, designers typically had to raise the detection thresholds, which would mean giving up some systems sensitivity. Note that when one increases the detection threshold, one basically decreases the range at which one can detect an incoming missile.
For purposes of the subject invention, clutter frequency is defined as the Doppler shift associated with fixed objects on the ground relative to the moving platform.
Also by way of definition, the clutter spectrum is defined in a Doppler sense as the maximum plus or minus Doppler shift frequency associated returns from ground objects. The reason that there is a clutter spectrum is that one has scatterers behind the platform and scatterers in front of the platform. Thus, one obtains a clutter spectrum that has both plus and minus Doppler shift. In one worst-case scenario involving a supersonic platform, one might have a clutter spectrum as wide as 4 kilohertz. The clutter spectrum is important because it defines the range of the Doppler frequency shifts one wants to reject.
By setting whatever filters are used to reject Doppler shifts below the speed of the aircraft, one can set the filters to at least reject as much as 4 kilohertz of Doppler shift clutter.